1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for collecting seismic data using a vertical hydrophone cable and, more particularly, to mechanisms and techniques for increasing a coupling of the hydrophones from a vertical hydrophone cable to the ground.
2. Discussion of the Background
Land seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where oil and gas reservoirs are located.
Traditionally, as illustrated in FIG. 1, a land seismic survey 100 that uses vertical hydrophone cables is performed in the following way. Plural hydrophones 102 are electrically connected to each other along a cable 104. A well 106 is dug into the ground 108 to accommodate the plural hydrophones.
After all the hydrophones have been deployed, one or more seismic sources are brought into the field and actuated to generate the seismic waves. The seismic waves propagate through the ground until they are reflected by various reflectors. The reflected waves propagate to the hydrophones, where a pressure change of the earth is recorded. However, if the coupling between the hydrophone and the dirt around the hydrophone is not good, the recorded data is of poor quality.
A hydrophone typically has a cylindrical shape and a small size. Thus, a coupling between the hydrophone and the well might be a problem when the hydrophone does not tightly fit into the well (supposing that the hydrophone is directly placed into the well). The coupling is improved if the diameter of the hydrophone is close to the diameter of the well, i.e., if the hydrophone is in tight contact with the walls of the well. However, obtaining a tight contact is a difficult problem. Thus, the coupling between the ground and hydrophone is traditionally poor, and also not well understood. The hydrophone-ground coupling may be defined as the difference between the pressure measured by the hydrophone and the pressure in the ground without the hydrophone. This definition is appropriate for designing a hydrophone.
However, once the hydrophone is designed and needs to be deployed, the practicing geophysicist has to deal with the fact that the hydrophone may not be appropriately deployed. For example, the hydrophone may not be “well” coupled to its surroundings. In this situation, the above definition might not be appropriate. For this situation, those skilled in the art would consider that a “bad” hydrophone coupling refers to the difference between the pressure as measured by the badly coupled hydrophone and the pressure as measured by the well-coupled hydrophone.
Irrespective of the used definition, the ground-hydrophone coupling is a persistent problem in the art because it is problematic to make the casing of the hydrophone to lightly contact the well and, at the same time, to ensure that the hydrophones are easily retrievable from the well when desired. One method known in the industry is to attach a cable 110 with a high mechanical resistance to the casing of each hydrophone and, when the time arrives to remove the hydrophones, to pull this cable up. However, if the well has collapsed at the location of one hydrophone, that hydrophone may be stuck at that position and even pulling the cable 110 may not retrieve the hydrophone.
Therefore, there is a need to improve the coupling of the hydrophone to the ground and at the same time to make easier and safer the process of retrieving the hydrophones.